As the most important manufacturing process in modern industry, machining is defined as the process of removing material from a workpiece in the form of chips. To perform the machining operation, relative motion is introduced between the tool and the workpiece. This relative motion is achieved in most machining operations by means of a primary motion, called cutting speed and a secondary motion, called feed. The shape of the tool and its penetration into the workpiece surface, combined with these motions, produce the desired shape of the resulting workpiece surface.
Common machining operations, such as drilling, turning, milling, and grinding, are capable of generating certain part geometries and surface textures. For example, the turning operation uses a cutting tool to remove material from a rotating workpiece to generate a cylindrical shape. As another example, grinding, which is the most precision machining process, generates smooth surfaces and fine tolerances.
In particular, grinding involves removing materials by creating a contact between a grinding wheel and a workpiece. Each grain of the grinding wheel removes a chip from the surface of the workpiece material and generates a surface finish. Material removal is done by individual grains whose cutting edge is bounded by force and path. The initial cutting interface is characterized by elastic deformation, followed by plastic flow of workpiece material. As discussed in J. Kopac and P. Krajnik, “High-performance grinding—A review,” Journal of Materials Processing Technology, Vol. 175, No. 1-3, pp. 278-284, 2006, which is hereby incorporated by reference, penetration between two hard materials influences the kinematics and contact condition.
A major limiting factor in any machining process is thermal damage caused by heat. In a machining process, energy is converted to heat, which is concentrated within the cutting zone. The high temperature produced can cause various types of thermal damage to the workpiece, such as burning, phase transformation, softening of the surface layer with possible rehardening, unfavorable residual tensile stresses, cracks, and reduced fatigue strength. To some extent, heat can also increase tool wear and reduce tool life.
Heat damage can be reduced by applying cooling fluid, also known as coolant, to remove the heat created by the interaction between the workpiece and the cutting tool and to lubricate the surfaces between them to reduce the amount of friction in the cutting zone. Because the coolant removes heat by way of conduction, the colder the fluid, the more effective the heat transfer. The fluid is also used to flush away chips. In addition, when the cutting fluid is applied to the cutting zone, it will initially undergo nucleate boiling. This process enhances the rate of heat transfer between the workpiece and the fluid.
There are four categories of cutting fluids based on composition, as suggested in K. Blenkowski, “Coolants and lubricants: part 1—the truth,” Manufacturing Engineering, pp. 90-96, 1993 and J. A. Webster and C. Cui, R. B. Mindek, “Grinding fluid application system design,” CIRP Annuals, Vol. 44, No. 1, pp. 333-338, 1995, both of which are hereby incorporated by reference for everything they describe. The publication by P. Q. Ge, L. Wang, Z. Y. Luan, and Z. C. Liu, “Study on service performance evaluation of grinding coolants,” Key Engineering Materials, Vol. 258-259, pp. 221-224, 2004, further shows that no fluid is perfect for all aspects of machining processes. This Ge et al. publication is also incorporated by reference for everything it describes. The significances of cooling, grinding forces, and thermal behavior have also been studied. In particular, it has been shown that water-based emulsions have better cooling effect, but generally lead to higher grinding forces.
Surface profile and roughness of a machined workpiece are two of the most important product quality characteristics and in most cases a technical requirement for mechanical products. Achieving the desired surface quality is of great importance for the functional behavior of a workpiece. Surface quality of a workpiece is generally indicated by surface roughness, surface physical and chemical performance, surface fluctuation, surface hardness, and residual stress.
Beyond machining processes, many mechanical or chemical systems also generate a significant amount of heat during their operations, due to frictions between components, combustions, or chemical reactions in the working zone. Cooling by way of cooling media or coolant such as gas or fluid is often needed to minimize thermal damage and maintain normal system performance in these systems.
Conventional cooling methods for reducing thermal damages include cryogenic cooling, spray cooling, air cooling, active cooling, megasonic cooling, actively cooled and activated cooling. The limitations of these conventional cooling methods are discussed below.
Cryogenic cooling utilizes a jet of liquefied gas such as liquid nitrogen. In this method, cooling is realized through a very high temperature gradient generated by contrast between the high temperature in the working zone and the very low temperature of the liquid nitrogen. The method has been shown to be effective in grinding ductile materials. However, for brittle materials, the very high temperature gradient may present a problem due to the possibilities in generating excessive thermal stresses on the surfaces of brittle materials. In addition, the method requires frequent replenishment of liquid nitrogen, which is uneconomical for long term use and requires great care for safety.
Spray cooling is a frequently used method of heat removal in many machining processes. However, it is not practical in precision machining processes.
In the air cooling method, the temperature is typically reduced to −10° C. ˜60° C. The temperature gradient is still quite large. However, in terms of specific heat and thermal conductivity, the physical properties of chilled air are more unfavorable than those of water based coolant. The delivery speed is generally up to 100 m/s, which is approximately 40-200 times of the one for water-based coolant, thereby causing a high level of noise.
In the active cooling method, an active cooling system is utilized to reduce the machining temperatures in the cutting zone through force convection. The active cooling system includes a coolant tank connected to an evaporator of the heat pump for heat exchange to remove the machining heat so as to reduce temperature in the working zone.
In ultrasonic or megasonic cooling, a floating nozzle having an integrated ultrasonic or megasonic transducer is utilized to provide coolant to cool the cutting zone. The surface quality improvement in the ultrasonic and megasonic cooling is attributed to the fluid cavitation effect. For example, previous studies have shown that megasonic cooling allows an increase in the grinding ratio by about 2 times and an improvement in the surface roughness by 20 to 30%. The temperature gradient mechanism was not utilized and as such cooling effectiveness improvement was limited.
In an actively cooled and activated coolant method, the cooling mist generated through a high frequency activation is able to take away heat from the cutting zone by way of evaporation effect. It has been shown that the actively cooled and activated cooling can achieve a 22.9% of average surface quality improvement in depth of cut tests and a 23.77% of average surface quality improvement in table speed tests. In these tests, an average improvement up to 36.68% in roughness value (Ra) has been obtained.
However, these conventional cooling methods are often insufficient to provide cooling necessary for producing high-quality workpieces. In particular, in conventional ultrasonic and megasonic cooling, there is a technical limitation in the piezoelectric activation component which imparts an upper limit in the activation strength, thereby limiting the cooling effect. In addition, it is also desired for a cooling system to have the capability to adjust the strength of cooling provided to the working or cutting zone so that the cooling effect is optimized for a given process.